standard/collection: add `Iterator::next_by` as a safe multiple `next`

Signed-off-by: Jean Privat <jean@pryen.org>

diff --git a/lib/standard/collection/abstract_collection.nit b/lib/standard/collection/abstract_collection.nit
index 2f7e454..c33416f 100644 (file)
--- a/lib/standard/collection/abstract_collection.nit
+++ b/lib/standard/collection/abstract_collection.nit
@@ -191,6 +191,37 @@ interface Iterator[E]
        # Require `is_ok`.
        fun next is abstract
 
+       # Jump to the next item `step` times.
+       #
+       # ~~~
+       # var i = [11, 22, 33, 44].iterator
+       # assert i.item == 11
+       # i.next_by 2
+       # assert i.item == 33
+       # ~~~
+       #
+       # `next_by` should be used instead of looping on `next` because is takes care
+       # of stopping if the end of iteration is reached prematurely whereas a loop of
+       # `next` will abort because of the precondition on `is_ok`.
+       #
+       # ~~~
+       # i.next_by 100
+       # assert not i.is_ok
+       # ~~~
+       #
+       # If `step` is negative, this method aborts.
+       # But specific subclasses can change this and do something more meaningful instead.
+       #
+       # Require `is_ok`
+       fun next_by(step: Int)
+       do
+               assert step >= 0
+               while is_ok and step > 0 do
+                       next
+                       step -= 1
+               end
+       end
+
        # Is there a current item ?
        fun is_ok: Bool is abstract